The Diabetes Prevention Trial - Type I (DPT) is a Multicenter, NIH sponsored study designed to test the hypothesis that intervention during the prodromal phase of Type I diabetes may delay its clinical onset. First degree relatives of patients with IDDM are screened for the presence of islet cell antibodies. Those testing positive undergo glucose tolerance tests. High risk patients who have diminished insulin secretory responses to glucose are randomized to a control, untreated group, and to a group that will receive a 4 day infusion of insulin followed by chronic subcutaneous ultra lente insulin. Follow-up visits will assess the state of glucose tolerance every six months. The results of the study will provide important information regarding the management of children and adults at risk for the development of Type I diabetes.